In printing technology it is known to print register marks for various purposes, for example for calibration purposes or for the adjustment of the circumferential register for a print job. As a rule, such register marks consist of a plurality of register lines, with each printing unit of the printing machine printing at least one register line within the register mark. As a rule, the register marks are directly printed on a circulating transport belt of the printing machine.
Subsequently, the register marks thus printed are moved past a register sensor that measures the register mark. This register sensor, as a rule, is only able to detect the start and the end of a respective register line based on light/dark or dark/light transitions. FIG. 2 shows an example of a register mark and an example of a signal curve of a register sensor in the case of an error-free detection of the register mark. An error-free detection is given whenever the signal curve of the register sensor indicates that a number of signals corresponding to the number of expected register lines exists above the detection threshold value. As a rule, at least twice as many signals than expected register lines will be present above the threshold value, because each time a signal is present at the start of the register line (transition from light to dark) as well as at the end of a register line (transition from dark to light).
If now, for example, a malfunction occurs in one of the printing units, said malfunction having the effect that one of the register marks is not being sharply or not with full intensity transferred to the transport belt, it is possible for the signal level of the register sensor to be located below the detection threshold value. The resultant signal curve would then not be consistent with the expected curve (there is not a sufficient number of signals above the threshold value), so that the register mark as a whole is discarded as being faulty. If only individual register marks are discarded, this does not represent a problem, as a rule. However, if this status persists for a certain period of time (e.g., for a few minutes), processes related to the printing of the register marks such as, for example, a calibration or the adjustment of the circumferential register of entire printing jobs can no longer be successfully performed, because no data are available therefore.
This status can be automatically recognized in a relatively simple manner, however, the localisation where the malfunction occurred is very complex and time-consuming. At this time, no information is being obtained as to the printing unit where the malfunction might have occurred because the entire information regarding the register mark is being discarded. Until now, only a manual process has been provided for localizing the malfunction. In this process, a service technician causes the register marks to be printed on the transport belt of the printing machine and interrupts this printing before the respective register marks are removed again by a cleaning device for the transport belt. Then, the service technician uses an adhesive tape to lift one or more register marks off the transport belt and attempts to visually determine which one of the color separation images could display the problem. As is readily obvious, this method is very time-consuming and fraught with errors.
Therefore, it is the object of the invention to automatically detect errors in individual color separation images of a multi-color printing machine in a simple manner.